Projection televisions are widely used in both the consumer (home entertainment) and commercial (presentation) markets. Instead of employing direct viewing of an image formed by electron beam impingement on the phosphors of a cathode ray tube, such televisions use a projection lens system to form an enlarged image of the face of one or more CRT tubes on a viewing screen.
For color systems, one three-color CRT and one projection lens system can be used or, more commonly, three CRTs and three projection lens systems are used, with each CRT/projection lens system combination producing an individual color image, e.g., red, green, and blue images, which are superimposed at the viewing screen to form a full color image. The projection television can be of the front or rear projection type depending on whether the viewer and the CRT/projection lens system(s) are on the same or opposite sides of the viewing screen.
A typical projection lens system contains at least three lens elements, at least some of which are typically composed of plastic materials, such as, acrylic or polyolefin. Plastic lens elements are desired both from a weight and cost perspective, especially when a particular lens element needs to have an aspherical surface to provide aberration correction. The projection lens system can be either air coupled or liquid coupled to the faceplate of the CRT. As the names imply, in an air coupled system, the interface between the CRT and the lens system, or more particularly, the last lens element of the lens system, is air, while in a liquid coupled system, it is a coupling fluid (coupling liquid).
The present invention is concerned with liquid coupled systems and, in particular, with providing improved coupling fluids for use in such systems. The requirements for an ideal coupling fluid are numerous. Among other things, the fluid should minimize reflection loss at the interface between the fluid and the CRT, should cool the CRT, and should generally improve the veiling glare of the system.
To date, ethylene glycol, mixtures of ethylene glycol and glycerol, and mixtures of ethylene glycol and water have been employed extensively as coupling fluids in projection TVs. The preferred fluid in recent years has been ethylene glycol/glycerol mixtures due to the relatively low volatility of the constituent components. As a result, the loss of coupling fluid through the last lens element of the projection lens system has been minimized. Also, the refractive index of an ethylene glycol/glycerol mixture is higher than pure ethylene glycol or a mixture of ethylene glycol and water. In addition, ethylene glycol/glycerol mixtures exhibit reasonable environmental friendliness, the ability to operate near 90.degree. C., low cost, good compatibility with molded optics, good thermal conductivity, and good optical clarity.
A preferred composition for coupling fluids of this type has been 80% ethylene glycol and 20% glycerol. Further addition of glycerol would cause the projected image to squirm, due to natural convection associated with thermal gradients in a viscous fluid.
Mixtures of ethylene glycol/glycerol, however, do have some properties which limit their usefulness. For example, the inherent electrical conductivity of this mixture is a problem. If an accidental spill occurs and the fluid gets onto a printed circuit board, the board has to be discarded to avoid electrical malfunction and even possible fire caused by the relatively high electrical conductivity of the fluid. Even more basically from an optics point of view, the refractive index of the mixture is lower than desired. Accordingly, couplers employing this mixture still suffer from relatively high levels of reflection at the CRT/coupler fluid interface. These reflections, in turn, lower the overall contrast of the projected image, making projection televisions less acceptable to some users in comparison to direct view televisions.